net/dpaa2: fix jumbo frame flag condition for MTU set

net/dpaa2: fix jumbo frame flag condition for MTU set

The jumbo frame uses the 'RTE_ETHER_MAX_LEN' as boundary condition,
but the Ether overhead is larger than 18 when it supports VLAN tag.
That will cause the jumbo flag rx offload is wrong when MTU size is
'RTE_ETHER_MTU'.

This fix will change the boundary condition with 'RTE_ETHER_MTU' and
overhead, that perhaps impacts the cases of the jumbo frame related.

Fixes: e16408499412 ("net/dpaa2: configure jumbo frames")
Cc: stable@dpdk.org
Signed-off-by: Steve Yang <stevex.yang@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>